1. Field of the Invention
The present invention relates to an image processing apparatus for reproducing a high quality image.
2. Related Background Art
A laser beam printer has become popular because of its high image quality and high speed. FIG. 3 shows a principle configuration of a prior art laser beam printer. In FIG. 3, an image input signal Vin represented by a bit ON/OFF level is amplified by a laser driver 10 where it is converted to an ON/OFF level signal LDR for driving a laser unit 11, which may be a semiconductor laser. A light beam emitted from the semiconductor laser passes through an optical system comprising a collimeter lens 12, a cylindrical lens 13, a focusing lens 14 and a mirror 15, and it is focused such that the spot diameter of the light beam is constant at any axial point on a surface of a drum 16. A scanner motor 18 driven by a scanner driver 17 rotates a 10-sided mirror 19 to linescan the laser beam axially of the drum. Thus, an electrostatic latent image is formed on the surface of the photoconductor drum in accordance with the input image signal. Toner (black) is left in this area in a subsequent development process so that the electrostatic latent image is visualized. A change of a potential on the surface of the drum correlates to an irradiation time of the laser beam and a beam intensity (power). In the laser beam printer described above, only a binary white-black image can be reproduced because the irradiation width per dot is constant and the beam intensity is constant. When a continuous density image such as a photograph is to be reproduced, gray levels may be reproduced by a dither method. However, since such an image is an aggregation of white and black dots, it is not sufficient to adequately represent the continuous density.